Normally wellbores for well and gas wells have a larger diameter or surface easing portion at the upper end that will be cased by a first string of casing. One or more strings of casing are subsequently installed with each string of casing being smaller in diameter. The first string of casing in a subsea well may be as large as 36″ in diameter. Typically, the operator jets or washes the first casing string into the seabed to a depth of about 400 feet. The operator accomplishes this installation by pumping fluid down the casing string to wash out the seabed as the casing is lowered. The surrounding formation settles in and around the casing string, holding it in place. The operator may also cement the first string of casing.
A subsea outer wellhead housing may be located at the upper end of the first string of casing on the sea floor. In other techniques, the first string of casing extends upward to a fixed platform above the sea level, and a wellhead housing is attached to the casing at the platform. The operator drills through the first string of casing to a second depth, then installs a second string of casing. The operator may repeat this process, installing a third or more strings of casing.
In some cases, having a longer first string of casing is desirable, such as one having a depth of about 1500 feet. A deeper first string of casing is helpful particularly for deep wells. However, increasing the depth of the first string of casing in subsea wells is not easy to achieve by jetting or washing down a large diameter string of pipe.
Techniques other than jetting or washing down surface casing are known. For example, operators normally install the first string of casing in land-based wells by first drilling the wellbore with a drill bit, then lowering the first string of casing string into the well and cementing it in place. The first string of casing for a land-based well is normally not as large as the first string of casing of a subsea well.
Although not typically done offshore, casing, include a first string of casing, may also be installed simultaneously as the well is being drilled. In this technique, the operator installs a bottom hole assembly at the lower end of a string of casing being made up. The bottom hole assembly includes a drill bit and a locking mechanism that locks the bottom hole assembly to the casing string for rotation in unison with each other. The operator grips the upper end of the casing string with a casing gripper. The top drive supports and rotates the casing gripper and the casing string, causing the drill bit to rotate to drill the well. When reaching a desired depth, the operator optionally may retrieve the bottom hole assembly while the casing remains in the well. The operator then cements the easing in place.
Casing while drilling becomes difficult in the case of very large diameter casing. One reason is that large diameter casing may not have the strength to transmit the necessary torque throughout its length. The friction between the large diameter casing string and the borehole sidewall can be high.
Mud motors are sometimes used in drill strings for causing rotation of the drill bit relative to the drill pipe. A mud motor operates in response to drilling fluid pumped down the drill pipe string. Mud motors are particularly useful for drilling horizontal or directional wells. Mud motors may also be installed in a bottom hole assembly of a casing drilling assembly. The reactive torque caused by the mud motor can be transmitted back to the casing string, which may be maintained in a non-rotating position. Rotating the casing string while casing drilling, however, is desirable to smear and condition the mudcake on the borehole walls. Thus, the operator typically will rotate the casing string at the same time the mud motor is operating. The casing will rotate in the same direction but at a slower speed than the mud motor, if so. The operator causes the casing string to rotate by rotating the casing gripper with the top drive.
During casing while drilling of land-based wells, the upper end of the string of casing will be located at the drilling rig and gripped and rotated by a casing gripper. Extending the upper end of the string of casing to the drilling rig during a casing drilling operation may not be feasible for an offshore well located in deep water. If the upper end of the casing string, once installed, is to be supported by a subsea wellhead assembly, the operator would not want the upper end of the casing string to extend any higher than the subsea wellhead assembly. Otherwise, the operator would have to unscrew each joint of casing extending above the subsea wellhead assembly, and the sea floor may be thousands of feet deep.
Liner drilling is another technique involving deploying a string of casing while drilling. A liner string is made up of joints of pipe that are the same as casing and which are cemented in the well. A difference is that the liner string extends only a short distance above the lower end of the previously installed casing string. Casing strings, cm the other hand, extend to the top of the well. In liner drilling, a selected length of casing is made up with a bottom hole assembly having a drill bit. The liner is deployed on a string of drill pipe, and rotation is imparted to the liner string by the string of drill pipe. The drill pipe may connect to the upper end of the liner string and transmit torque through the liner string to the bottom hole assembly. Alternately, the drill pipe may extend concentrically within the liner string to the bottom hole assembly. The liner string is mounted to the drill pipe for rotation with the drill pipe, thus some of the torque would pass through the liner string and some through the drill pipe to the bottom hole assembly.